Concerted and stepwise mechanisms in the DielsAlder and Michael reactions of furans with methyl 3-nitroacrylate Experimental and theoretical studies
Bibliographic record
Abstract
The DielsAlder reactions of the furans (1a1c) and methyl 3-nitroacrylate (2) produce the cycloadducts via a concerted mechanism. Then, the products subsequently undergo retro-DielsAlder reactions to give the Michael adducts via a stepwise mechanism. We have used both experimental and theoretical methods to investigate the mechanism for, and the regio- and stereo-selectivity in, the DielsAlder and Michael reactions. To account for the selectivity observed in these additions, we examined the frontier molecular orbitals involved, and to locate the transition states (TS), we employed a B3LYP/6-31G* level density functional calculation. We were able to explain the regio- and stereo-selectivity of the four possible DielsAlder adducts observed in the experiments by a comparison of their calculated activation energies, which ranged from 11 to 18 kcal/mol (1 cal = 4.184 J). The s-cis forms of the dienophile (2) were found to be more stable than the s-trans forms, both in the ground state and in the transition state. In addition, the endo transition structures for the ester group exhibited a lower energy barrier by 0.3 kcal/mol than their exo counterparts, a value that is in disagreement with the experimental results. The presence of the nitro group in 2 may play an important role in determining the selectivity. Our attempts to find a stepwise mechanism leading to the DielsAlder adduct via a zwitterion were unsuccessful. We did find that two stepwise processes lead to the formation of Michael adducts generated via an electrophilic attack by 2 at the α position of 1b and 1c, and an intramolecular proton-transfer mediated by the formation of a four-membered structure. The potential energies for these reactions showed values in the following ranges: 17 kcal/mol for 5a and 5b, and 11 to 12 kcal/mol for 7a and 7b for the first step, and, for the proton-transfer step, 4851 kcal/mol for 5a and 5b, and 4145 kcal/mol for 7a and 7b. Solvent effects in chloroform on the Michael addition did not appear in the electrophilic attack step (less than 4 kcal/mol), but the transition state of the later process was stabilized by 613 kcal/mol.Key words: DielsAlder reactions, Michael reactions, density functional method, furan, retro-DielsAlder reactions, solvent effect.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.001 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".